Conventional commercial solar thermal power generation technologies mainly utilize trough/tower/dish solar collectors for collecting thermal energy and then utilizes a Rankine cycle or a Stirling cycle for converting thermal energy collected by the collectors into electric energy. A trough solar thermal power generation system and a tower solar thermal power generation system are used in a large scale and usually employ a Rankine cycle for power generation. A dish-type solar thermal power generation system usually puts a Stirling engine at a focal point of a dish collector and employs a Stirling cycle for power generation because of high concentration ratios, high heat collection temperatures and relatively small scales.
A modern thermal power plant generates power mainly by employing a Rankine cycle which is an ideal thermodynamic cycle process. In trough solar thermal power generation system and the tower solar thermal power generation system heat a heat exchange fluid (for example, air, heat-conducting oil, molten salts etc.) by employing a light-focusing device, and then utilizes the heat exchange fluid for heating a working medium in Rankine cycle, thereby facilitating conversion of heat into power through Rankine cycle. In Rankine cycle, feedwater is heated through an external heat source (for example, a high-temperature flue gas), which is an effective measure for increasing output work.
Stirling cycle is a reversible cycle consisting of two constant-volume endothermic processes and two constant-temperature expansion processes. A heat engine absorbs heat from a high-temperature heat source during high-temperature expansion and releases heat to a low-temperature heat source during low-temperature compression. A Stirling engine realizes conversion from heat to power by employing Stirling cycle, the efficiency thereof is related to the temperature of a low-temperature heat source. The lower the temperature of the low-temperature heat source, the higher the efficiency of the Stirling engine is. Therefore, measurements of effectively taking away thermal energy of the cold chamber of the Stirling engine and reducing temperature of the low-temperature heat source of the Stirling engine help to improve the heat efficiency of Stirling cycle.
Solar thermal power generation technologies capable of facilitating commercial large-scale power generation at present are divided into trough/dish/tower solar thermal power generation technologies according to different concentrator types.
The trough solar thermal power generation system gathers sunlight onto a heat collection pipe which is at the focal line by employing a parabolic reflector. The trough solar thermal power generation system, which usually tracks sun in a single-axis one-dimensional way, has a concentration ratio of 40-80, a heat collection temperature usually not exceeding 400° C. and a fixed heat collection focal line, and is easy for large-scale commercialization. Since the concentration ratio of the parabolic reflector is small and the heat collection temperature is low, the trough solar thermal power generation system usually employs Rankine cycle for power generation, concretely utilizes a trough system to obtain a working medium with thermal energy, then generates high-temperature steam through the working medium with thermal energy and drives a steam turbine generator unit for power generation. During solar thermal power generation Rankine cycle, the trough solar thermal power generation system is low in solar to electric conversion efficiency because of low collector field efficiency and low heat collection temperature.
The dish solar thermal power generation system utilizes a rotary parabolic reflector to gather sunlight onto a heat collector at a focal point of the rotary parabolic reflector. The dish solar thermal power generation system, which usually tracks sun in a double-axis two-dimensional way, has a concentration ratio of 3000 and accordingly has a heat collection temperature of 900-1200° C. Since the concentration ratio of the rotary parabolic reflector is large and the heat collection temperature is high, Stirling cycle is utilized for obtaining a relatively high solar to electric conversion efficiency. A conventional dish solar thermal power generation system usually directly disposes a small-size Stirling engine at the focal point for facilitating power generation, however the solar thermal power generation system hardly achieves large-scale power generation because of limit of the size of the reflector surface and the fact that the Stirling engine moves along with movement of the rotary parabolic reflector automatically tracking sunlight. Meanwhile, the cold chamber of the small-size Stirling engine dissipates heat more difficultly. Additionally, during solar thermal power generation Stirling cycle, the Stirling engine needs a cooling device to take away the heat generated when it works, and this thermal energy is wasted since it is not utilized.